The present invention relates to the general art of wireless communications and particularly radio navigation and radio localization.
Localization of remote wireless terminals by a satellite system is quite useful in many cases, particularly while being out of the coverage area of terrestrial cellular networks. This requirement is especially relevant in case of emergency, when the position of the terminal can mark the place of a person in distress, either at sea, on the land and even in the air.
Thus, many satellite systems enable localization of compatible terminals, such as satellite mobile phones or data terminals operated by: Iridium, Inmarsat, Globalstar, Thuraya and Orbcomm; and especially dedicated Search and Rescue (SAR) satellite systems, particularly Cospas-Sarsat and its dedicated terminals—radio beacons.
Normally, the localization criteria to evaluate navigation and localization satellite systems are: coverage area, availability and accuracy. State of the art SAR satellite systems, especially Cospas-Sarsat, provide, or will provide in the near future, worldwide coverage, continuous service, 1-2 sec latency and about 100 m accuracy, based on a large constellation of satellites and terrestrial base stations.
While that is an impressive benchmark, it should be recalled that SAR systems are often challenged by extreme environmental or human conditions that badly affect people and devices. For example, localization of a person fallen overboard a ship in a stormy night can be very difficult if located at no better than 100 meters error; a ship tilted and rolled by high seas cannot be expected to have its beacon antenna properly oriented to the satellites; and an airplane crash could badly damage the antenna beacon installed onboard, reducing the number of satellites it can access.
Thus, it is one fundamental object of the present invention to improve the localization accuracy of satellite systems, and enable localization at degraded conditions such as fewer satellites in view.
The present invention is particularly applicable to the international Cospas-Sarsat Search and Rescue satellite system, and though the scope of the invention is much wider, the invention will be sometimes described in terms of the Cospas-Sarsat system, to better explain the invention and some of its embodiments. That definitely does not mean that the present invention is limited to a specific system or application.
A radio beacon is a device that allows tracking a moving object such as ship, aircraft, person, animal or asset via a remote receiver/detector. Depending on the beacon, particularly its transmission frequency and power, the tracking range can be as short as some meters or practically worldwide, in case of satellite served beacons.
Distress radio beacons, also known as emergency beacons: EPIRB (Emergency Position Indicating Radio Beacon) for vessels, ELT (Emergency Locator Transmitter) for airplanes, and PLB (Personal Locator Beacon) carried by individuals, are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. In particular, EPIRBs, ELTs and PLBs are served by the worldwide system of Cospas-Sarsat, an international satellite system for search and rescue. When manually activated, or automatically activated (upon immersion or collision), such beacons send out periodic distress signals that are monitored worldwide by the system satellites, and their position is informed to Rescue Coordination Centers (RCC) that coordinate the actual rescue. Presently (2016), almost any country operates an RCC, and Cospas-Sarsat satellites cover the entire world.
The Objective of the Cospas-Sarsat system is to reduce, as far as possible, delays in the provision of distress alerts to SAR services, and the time required for locating a distress and providing assistance, which have a direct impact on the probability of survival of the person in distress at sea or on land.
Presently, the Cospas-Sarsat system is comprised of a SAR segment based on LEO (Low Elevation Orbit) satellites, named LEOSAR, and another segment based on GEO (Geostationary) satellites, named GEOSAR. A SAR segment based on MEO (Medium Elevation Orbit) satellites, named MEOSAR, is in development, planned to be fully operational by 2018; these MEO satellites are part of the GPS, Galileo and Glonass constellations. The Cospas-Sarsat system also comprises base stations that detect the beacon signal relayed by the satellites, determine the beacon position and notify the RCC. The LEOSAR base station is named LEOLUT (LEO Local User Terminal), GEOSAR base station is named GEOLUT and the MEOSAR base station is named MEOLUT.
A Cospas-Sarsat beacon, when activated, periodically broadcast short bursts at 406 MHz, more or less every 50 seconds. These transmissions are modulated with a short message, typically 144 bits long, indicating the beacon unique ID and optionally also self-position acquired by a built-in GNSS receiver. The LEOSAR provides also an independent/autonomous method of localization of the beacon, based on the Doppler Effect, and the MEOSAR provides independent localization based on TOA (Time of Arrival) and FOA (Frequency of Arrival) measurements. One of the major objectives of the MEOSAR is to enable independent localization of the beacon upon a single burst, in order to promptly determine the distress location, especially in airborne applications, before the beacon might get out of order, e.g. upon crashing. However, the MEOLUT, when applying TOA and FOA localization, requires several satellites in common view to relay the beacon burst, typically 3 but not less than 2 in order to the determine the beacon position. Now, though many such satellites are planned to be launched and deployed in few years, the requirement for multi satellites simultaneously in view is still a limit, especially before the full constellations are deployed, but also later at less than optimal conditions, e.g. when a PLB broadcast from a deep canyon or creek, or an EPIRB is extremely tilted at high seas, or an ELT antenna damaged upon emergency landing, or Man overboard with PLB often immersed in the water.
A particular situation that the present invention addresses is the notorious Man overboard (MOB) or Person overboard (POB) accident. Man overboard is a situation in which a person has fallen from a boat or ship into the water and is in need of rescue. People may fall overboard for many reasons: they might have been struck by a part of the ship; they might lose their footing due to a slippery deck or an unexpected movement of the boat, or washed off board by a huge wave. Falling overboard is one of the most dangerous and life-threatening things that can happen at sea. This is especially dangerous with short-handed small boats and particularly single-handed, and in combination with self-steering gear it could be fatal. Thousands of people are lost at sea every year due to MOB. Fast detection and location of such accidents is crucial since survival time in water is short, typically under 6 hours at 10° C.
Technology can be used to assist in the retrieval of people who fall overboard. Many GPS chart plotters designed for marine use have a Man Overboard button (MOB). This button is to be pushed as soon as a Man Overboard alarm is raised, causing the plotter to record the actual position of the ship, i.e. latest known position of the person overboard, as a waypoint to which it is possible to return after a short maneuver.
Several manufacturers make man overboard alarms which can automatically detect a man overboard incident. The hardware consists of individual units worn by each crew member, and a base unit installed onboard. Some systems are water activated: when an individual unit comes in contact with water, it sends a signal to the base unit, which sounds the man overboard alarm. Other automatic detection systems rely on a constant radio signal being transmitted between an individual unit and the base unit; passing outside the transmission range of the individual unit and/or falling into the water causes the radio signal to degrade severely, which makes the base unit sound the man overboard alarm.
Yet, present MOB alert devices, also known as Marine Survivor Locating Devices (MSLD), cannot be located at more than a couple of miles away from the mother ship, so as the victim drifts away, even the onboard record of last known position of MOB becomes obsolete. At high seas and low visibility conditions, locating a MOB becomes a significant challenge.
In addition, MSLDs typically do not employ GPS, since it is not efficient due to the fast and unexpected nature of MOB accidents, and the fact that almost all the victim body is typically in the water, and his/her hands (in case of wrist worn MSLD) are moving, and its antenna is not necessarily in view with the satellites, particularly difficult to detect three satellites for 30 seconds to fix a GPS position.
Indeed, some modern MSLDs employ AIS (Automatic Identification System) or DSC (Digital Selective Calling) transmitters, practically because many vessels are already equipped with AIS or DSC receivers, however the location data provided by AIS and DSC transmitters is based on GPS embedded receivers, and if those cannot fix the MSLD position then the device cannot be localized over AIS or DSC.
It is then another object of the present invention to increase the probability of localization of Marine Survivor Locating Devices (MSLD), especially in severe environmental conditions and particularly in MOB situations.
Other devices in the market, generally called SEND (Satellite Emergency Notification Devices), such as SPOT on the Globalstar satellite constellation, are also used for localization upon distress, yet based on commercial satellite systems. So SEND systems are also relevant to the present invention.
Time of Arrival (TOA) is a well-known localization method in which a receiver calculates its distance to a transmitter based on the time it takes a signal to travel between the transmitter and the receiver, and multiplying the travelling time by the signal propagation speed, usually the speed of light. Theoretically, calculating distances to three transmitters, and knowing these transmitters spatial coordinates, enables a receiver to calculate its own spatial coordinates, by resolving three quadratic equations based on the three dimensional Pythagorean theorem. This method, also known as trilateration, is the basic algorithm employed by Global Navigation Satellite Systems (GNSS) such as the US GPS, Galileo and Glonass. The geometric representation of this method is of three spheres, each sphere having a transmitting satellite at its origin and a radius of the relevant TOA multiplied by the speed of light; these spheres intersecting at two common points, on which of these the receiver is positioned. A similar method is also applied in the Cospas-Sarsat MEOSAR, for localization of a radio beacon. In the MEOSAR, the satellites are installed with receivers, and a radio beacon is localized by measuring the TOA of a signal emitted from the beacon and detected by three of these receivers; actually, the satellites are installed with relays or re-transmitters acting as “bent pipes”, i.e. instantaneously relay the beacon signal (converted to a different frequency) to the MEOLUT base station at which the trilateration calculation is performed. FIG. 1 illustrates that method. However, since the beacon transmission time is unknown at the MEOLUT, time measurements of a beacon signal relayed by three satellites provide three TOA equations with 4 unknowns: the beacon (x, y, z) coordinates and the beacon transmission time (tx), provided that the satellites and MEOLUT positions are known, as well as the relayed signal detection time (tr1, tr2 and tr3 respectively). So practically, three time measurements on the beacon signal enable resolving just the beacon 2D (x, y) position, e.g. Latitude and Longitude, assuming that the altitude is known or estimated, as could be applied for example in case of ocean sailing ships. For 3D (x, y, z) position resolution another equation is required, possibly provided by a fourth satellite.
Another way to express the beacon position is by equations that do not include the beacon transmission time, but considering the difference in detection time of the same beacon signal relayed by two satellites. This format is known as TDOA (Time Difference of Arrival), and its 2D geometric expression, i.e. locus on which the beacon should be, is a hyperbola, compared to a circle in 2D TOA, as shown in FIG. 2. Still, a TDOA equation requires two satellites compared to a single satellite for TOA, so detecting the beacon signal relayed by 3 satellites, without knowing the exact transmission time instant, enables only 2D localization of the beacon, either through 3 TOA or 2 TDOA equations, as shown at the bottom of FIG. 1 and FIG. 2. It might appear from FIG. 1 that even 2 TOA circles enable 2D localization (2 points) however if the transmission time is unknown so are the origin and radius of the representing circle. For 3D localization based on time measurements, 4 relaying satellites are required, providing 4 TOA equations with 4 unknowns or 3 TDOA equations with 3 unknowns.
Yet, another type of measurement is employed at present art MEOLUTs contributing to the localization of the beacon: FOA (Frequency of Arrival). If the beacon transmission frequency is known, and the detection frequency is measured (even a relayed converted frequency), and if the satellite position and velocity at the time of relaying the beacon signal are known, then based on the Doppler effect another equation can be compiled for every signal relayed by a satellite and detected at the MEOLUT, as shown in FIG. 3. The geometric expression of that Doppler measurement, i.e. the locus on which the beacon should be, is a cone in 3D and a triangle in 2D (avoiding the base in both cases). Still, if the transmission time of the beacon is unknown, the satellite position and velocity at the time of relaying the beacon signal are also unknown, but can be expressed based on said time.
So, state of the art MEOLUTs can localize a beacon in 3D upon a single burst transmission relayed by as few as 2 satellites in view, by compiling and resolving 4 equations with 4 unknowns (the beacon coordinates and transmission time), expressing 2*TOA and 2*FOA measurements. With only 1 satellite in view, state of the art MEOLUTs cannot localize standard Cospas-Sarsat beacons even in 2D. It is then also an object of the present invention to enable localization of a beacon at the MEOLUT with only 1 satellite in view, particularly upon a single beacon burst transmission.
U.S. Pat. No. 7,522,639 by D. Katz, discloses a method for SYNCHRONIZATION AMONG DISTRIBUTED WIRELESS DEVICES BEYOND COMMUNICATIONS RANGE. Katz discloses communication devices configured to communicate with each other, which are usually dormant in order to save battery power, yet can simultaneously wake up and perform communication, upon sensing a same external event. That method, though very instrumental in certain scenarios, is difficult to apply worldwide such as required from distress beacons. In particular, there are not many practical options for generating an external event that could be sensed by and trigger beacons deployed in the middle of the ocean. Satellites do provide such an option but require a receiver at the beacon, which is not in the scope of U.S. Pat. No. 7,522,639.
A method enabling localization of a beacon at the MEOLUT with only 1 satellite in view is disclosed in U.S. patent application Ser. No. 14/465,872 filled by the present inventor, D. Katz, titled: Tracking a Radio Beacon from a Moving Device. Katz discloses there: “ . . . preferably, the transmission time instants and receiving time instants may be determined with respect to a same time reference, such as the GPS time. In this case, the time transmission may be encoded in reference to the last rising edge of the GPS 1 PPS (pulse per second) signal”. For example, as shown in FIG. 4 of the present invention the beacon is configured to transmit a burst exactly at the 1PPS epoch, acquired by a GNSS receiver comprised in the beacon. The MEOLUT that also has access to this 1PPS signal, measures the difference in time between the detection time of the (possibly relayed) beacon signal and the previous closest 1PPS signal, and multiplying this time measurement by the speed of light (C) provides the distance (i.e. pseudorange) travelled by the signal from the beacon to the MEOLUT. The satellite relay is considered to be “bent pipe”, however adding a certain delay, typically less than 1 micro second, which can be estimated and subtracted from said overall time measurement.
However, the 1PPS signal indicating the accurate GPS TIME is acquired at a GNSS receiver only upon solving the navigation equations, i.e. fixing self-position, and for that at least 3 satellites in view are required. Now, if the navigation satellites (GNSS) used by the beacon to acquire self-position and accurate time are different from the satellites used to relay the transmitted signal to the base station, as is the case in the older Cospas-Sarsat segments—LEOSAR and GEOSAR, then it is possible that a beacon will have 3-4 navigation satellites in view, acquire the 1PPS and use a single SAR satellite to relay a distress signal to a base station (LEOLUT or GEOLUT); however, in the MEOSAR the same satellites—GPS, Galileo and Glonass carry both GNSS and SAR payloads, so any satellite in view provides both GNSS and SAR services, just that for SAR the satellite should be in common view by beacon and MEOLUT. Nevertheless, the total number of satellites involved in the beacon localization in present art systems is not less than 2 upon a single beacon burst; sometimes a single satellite can be employed but at several positions, i.e. using several bursts, e.g. as disclosed by U.S. patent application Ser. No. 14/465,872.
Synchronizing the transmission to a globally acquired clock, such as GPS TIME, which can be also acquired independently at the receiver, basically enables the receiver to determine the transmission time of a beacon and use TOA equations; so if, for example, the beacon transmits a burst at the rising edge of the 1 second GPS TIME clock, as illustrated in FIG. 4, the receiver measures the delay between the relayed beacon signal detection time and the rising edge of the last 1PPS pulse acquired from the GNSS and that is considered the TOA. However, this method obtains some ambiguity, since the receiver does not know at which specific 1PPS pulse the beacon transmitted. The fact that the receiver refers to the last 1PPS pulse is based on the assumption that a more distant pulse means additional 1 sec of travelling, equivalent to additional 300,000 Kms of travelling, which is not expected with satellites orbiting some 20,000 Kms above the earth. Yet, in certain cases it is desirable to synchronize the transmission to a faster clock, e.g. 1 KPPS, and that could introduce an ambiguity which is more difficult to remove.
So it is yet another object of the present invention to synchronize TOA measurements to a globally acquired clock pulse, even with short period such that result in ambiguous TOA expressions.
GNSS signals, as known to the skilled person, report the transmission time instant, which is very instrumental in resolving the navigation equations. Cospas-Sarsat beacons, however, do not report that time, probably due to bandwidth limitations and lack of accurate built in clock. Furthermore, Cospas-Sarsat specifications call for a pseudo random pattern of transmission time intervals in order to decrease the probability of transmission collisions among a plurality of beacons. Theoretically, a beacon could transmit at pseudo random time intervals and still synchronize its transmissions to a global clock such as the 1 KPPS, however that is impossible when this clock is not acquired at the beacon, due to lack of navigational satellites in view, for example. Thus, it is still an object of the present invention to enable TOA measurements even if the exact transmission time of the beacon is unknown at the MEOLUT; it is still another object of the present invention to enable localization of a beacon even if no globally referenced time signal is available at the beacon, such as GPS TIME (1 PPS) or UTC (Universal Time Coordinated) or GMT (Greenwich Mean Time).
When enough satellites are in common view by the beacon and MEOLUT, enough time based and frequency based equations can be compiled at the MEOLUT to resolve the beacon coordinates; still, as known in the art, more measurements enable more equations based on the same unknowns, and having more equations than unknowns enable reducing the standard deviation error, i.e. more accurate localization. In this context, it is also an object of the present invention to enable more accurate localization of a beacon based on a certain number of satellites in view.
Other objects and advantages of the invention will become apparent as the description proceeds.